1. Field of the Invention
The present invention relates to a catalyst for purifying exhaust gases exhausted from an internal combustion engine of an automobile and, more particularly, an exhaust gas purifying catalyst for removing effectively high concentration hydrocarbons exhausted in engine start-up.
2. Description of the Related Art
In the prior art, for the purpose of purifying the exhaust gases from the internal combustion engine of the automobile, etc., the three-way catalyst that can perform oxidation of carbon monoxide (CO) and hydrocarbons (HC) and reduction of nitrogen oxides (NOx) simultaneously has been widely employed.
A large amount of HC, which is called xe2x80x9ccold HCxe2x80x9d, are exhausted at the low temperature in the engine start-up. However, since the exhaust gas temperature must be kept at more than 300xc2x0 C. to make the above three-way catalyst active, the three-way catalyst is inactive at the low temperature immediately after the engine start-up. Therefore, if only the three-way catalyst in the prior art is employed, the cold HC is not purified but exhausted as it is.
In recent years, for the purpose of purifying this xe2x80x9ccold HCxe2x80x9d, the HC adsorbing catalyst employing the zeolite has been developed.
The HC adsorbing catalyst is such a catalyst that adsorbs and holds temporarily the cold HC in the engine start-up during when the three-way catalyst is not activated, and then desorbs HC gradually to purify HC by using the three-way catalyst when the three-way catalyst is activated after the temperature of the exhaust gas is increased.
In the exhaust gas, there are various HCs having different molecular weights. Since the HC adsorbing catalyst using the zeolite has a correlation between an HC distribution in the exhaust gases and a pore diameter in the zeolite, the zeolite having the optimum pore diameter must be employed.
In the prior art, the pore diameter distribution is adjusted by blending the MFI-type zeolite as the main element with the zeolite (e.g., USY type) having another pore diameter. However, since the distortion of the pore diameter and the adsorption/ desorption characteristic is different after long time use according to the zeolite type, adsorption of the HCs in the exhaust gas becomes insufficient.
In the prior art, there are a three-way catalyst in which the noble metals such as platinum (Pt), palladium (Pd), rhodium (Rh), etc. are contained in the same layer, and another three-way catalyst in which the Rh layer and the Pd layer are coated separately, etc. Japanese Laid-Open Patent Publication Hei 2-56247 published in 1990 discloses an exhaust gas purifying catalyst in which the second layer containing the noble metals such as Pt, Pd, Rh, etc. as the main component and having the three-way catalytic function is provided on the first layer containing the zeolite as the main component.
Also the above HC absorbing catalysts are disclosed in Japanese Laid-Open Patent Publications Hei 6-74019 published in 1994, Hei 7-144119 published in 1995, Hei 6-142457 published in 1994, Hei 5-59942 published in 1993, and Hei 7-102957 published in 1995.
The three-way catalysts are strongly affected by the exhaust gas temperature and the air-fuel ratio (A/F). In order to cause these three-way catalysts to effectively serve their purifying function, the air-fuel ratio must be set in the vicinity of a stoichometric air-fuel ratio (A/F=14.6) at which the oxidation of HC and CO and the reduction of NOx are balanced. However, in the exhaust gas purifying catalyst in which the three-way catalyst layer is provided on the zeolite layer having the HC adsorbing function, since HC which has been adsorbed in the low temperature zone of the exhaust gas immediately after the start of the internal combustion engine is gradually desorbed with the increase of the exhaust gas temperature, the exhaust gas becomes fuel-rich. Therefore, the three-way catalyst cannot fulfill sufficiently the purification function, the purification of HC, CO, Nox cannot be achieved with good balance. As a result, if the HC desorption speed from the zeolite layer is too early, the effective HC purification cannot be achieved.
In addition, in the prior art, in the exhaust gas purifying catalyst that has the three-way catalyst layer on the zeolite layer, no particular study of a ratio of respective layer thickness has been made. However, according to the study made by the inventor of the present invention, it is found that, in the HC adsorbing catalyst that has the multi-layered structure consisting of the zeolite layer and the metal-based catalyst layer, the HC adsorption/desorption/purification cycle cannot be effectively carried out if the structure of respective layers is not proper.
In view of these subjects in the prior art, it is an object of the present invention to provide an exhaust gas. purifying catalyst capable of purifying effectively HC in the exhaust gas from the internal combustion engine at the low temperature immediately after the engine start-up.
A first aspect of the present invention provides an exhaust gas purifying catalyst, comprising a monolithic support including a plurality of cells whose sectional shape is an almost regular N-polygon (N is a natural number of more than or equal to 3), a hydrocarbon adsorbent layer containing zeolite as a main component formed on the monolithic support, a metal-based catalyst layer containing at least one type noble metal selected from the group consisting of platinum, palladium, and rhodium formed on the hydrocarbon adsorbent layer. Then, the hydrocarbon adsorbent layer is formed to satisfy a ratio Rc/Rf in a cell sectional shape that is set less than or equal to 1.7, where Rc is a distance from a center of gravity to an inner wall surface of the hydrocarbon adsorbent layer along a line extending from the center of gravity of the regular N-polygon in the cell sectional shape to a corner of the N-polygon, and Rf is a distance from the center of gravity to the inner wall surface of the hydrocarbon adsorbent layer along a line extending from the center of gravity perpendicularly to respective sides of the regular N-polygon.
It is preferable that, if the N-polygon is an almost regular triangle (N=3), the ratio Rc/Rf of the distance Rc to the distance Rf is set to satisfy Rc/Rfxe2x89xa61.6.
It is preferable that, if the N-polygon is an almost regular quadrangle (N=4), the ratio Rc/Rf of the distance Rc to the distance Rf is set to satisfy Rc/Rfxe2x89xa61.3.
The value of the ratio Rc/Rf tends to increase as the film thickness of the HC adsorbent layer becomes thinner. In the case that the ratio Rc/Rf is larger than 1.7, desorption of HC becomes quick if the thickness of the HC adsorbent layer is too thin, so that the sufficient function as the HC trapper cannot be performed.
A second aspect of the present invention provides an exhaust gas purifying catalyst, comprising a monolithic support having a plurality of cells whose sectional shape is an almost regular N-polygon (N is a natural number of more than or equal to 3), a hydrocarbon adsorbent layer containing zeolite as a main component formed on the monolithic support, and a metal-based catalyst layer containing at least one type noble metal selected from the group consisting of platinum, palladium, and rhodium formed on the hydrocarbon adsorbent layer and n, and wherein each cell sectional structure satisfies the following
1xe2x89xa6(Xxe2x88x92Rc)/(Yxe2x88x92Rf) less than 40,
where X is a distance from a center of gravity of the regular N-polygon to corners of the regular N-polygon, Y is a shortest distance from the center of gravity to cell sides of the regular N-polygon, Rc is a distance from a center of gravity to an inner wall surface of the hydrocarbon adsorbent layer along a line extending from the center of gravity of the regular N-polygon in the cell sectional shape to a corner of the N-polygon, and Rf is a distance from the center of gravity to the inner wall surface of the hydrocarbon adsorbent layer along a line extending from the center of gravity perpendicularly to respective sides of the regular N-polygon.
The value of (Xxe2x88x92Rc)/(Yxe2x88x92Rf) exhibits a tendency to reduce as the film thickness of the HC adsorbent layer becomes thicker. If the ratio (Xxe2x88x92Rc)/(Yxe2x88x92Rf) is more than or equal to 40, the thickness of the HC adsorbent layer is too thin and thus desorption of HC becomes quick. Hence, the hydrocarbon adsorbent layer cannot sufficiently perform the function of the HC trapper. In contrast, if the ratio (Xxe2x88x92Rc)/(Yxe2x88x92Rf) is less than 1, the thickness of the hydrocarbon adsorbent layer is too thick and thus the cavities in the center of the cells are blocked. Hence, the pressure of the exhaust gases passing through the cavities of the cells is increased. As a result, since the HC desorption is accelerated, the HC trapping function of the hydrocarbon adsorbent layer is disturbed.
Therefore, According to the second aspect of the present invention, since the relative film thickness of the hydrocarbon adsorbent layer can be controlled in respective cells by setting the ratio (Xxe2x88x92Rc)/(Yxe2x88x92Rf) within the appropriate range, the exhaust gases purifying catalyst which can suppress the desorption of HC and achieve the good HC trapping function can be provided.